The major object of this project is to uncover enzymatic steps involved in various genetic rearrangement reactions and study the mechanism of action of the enzymes involved. The reactions studied include: the site specific recombination of bacteriophage lambda, resolution of the intermediate of general genetic recombination, and transposition-replication reaction of bacteriophage Mu. The most recent developments include the establishment of an in vitro reaction system for the study of replicative transposition of bacteriophage Mu. This in vitro reaction yields both cointegrate products as well as simple insertion products and requires both A and B gene products of phage Mu along with other bacterial proteins. The reaction requires a substrate DNA molecule that carries two ends of Mu in their proper relative orientation. These conditions are also the requirements for the Mu transposition-replication reaction in vivo. Thus, the in vitro reaction observed is the authentic Mu transposition. This is the first time a transposition reaction has been demonstrated in a cell-free system.